Undefined offset in while loop with 2 conditions

I have an array like this :

array (size=3)
 0 => 
    array (size=2)
      'score' => float 17.891873624039
 1 => 
    array (size=2)
      'score' => float 17.883449353824
 2 => 
    array (size=2)
      'score' => float 4.702030124427

and have a while loop like this:

 $offset=1;
 while($scoreList[$rank+$offset]!=NULL && $scoreList[$rank]["score"]==$scoreList[$rank+$offset]["score"])
 {
    //do something
    $offset++;
 }

I need to check if the next index in the array exists and at the same time check if it is equal to the current object but I get

Undefined offset:4

I cannot put the second condition in an if statement because offset should be increased if these two indexes are equal so it falls in an infinite loop if I put it in if clause.